HB-EGF was first identified in the conditioned medium of cultured human macrophages. It is synthesized as a transmembrane, biologically active precursor protein (proHB-EGF) composed of 208 amino acids, which is enzymatically cleaved by matrix metalloproteinases (MMPs) to yield a 14-20 kDa soluble growth factor (sHB-EGF). Pro-HB-EGF can form complexes with other membrane proteins including CD9 and integrin α3β1; these binding interactions function to enhance the biological activity of pro-HB-EGF. ProHB-EGF is a juxtacrine factor that can regulate the function of adjacent cells through its engagement of cell surface receptor molecules.
sHB-EGF is a potent mitogenic and chemoattractant protein for many types of cells. Similar to all members of the EGF family, HB-EGF binds to the “classic” or prototypic epidermal growth factor receptor (EGFR; ErbB-1). However, while the mitogenic function of sHB-EGF is mediated through activation of ErbB-1, its migration-inducing function involves the activation of ErbB-4 and the more recently described N-arginine dibasic convertase (NRDc, Nardilysin). This is in distinction to other EGF family members such as EGF itself, transforming growth factor (TGF)-α and amphiregulin (AR), which exert their signal-transducing effects via interaction with ErbB-1 only. In fact, the NRDc receptor is totally HB-EGF-specific. In addition, unlike most members of the EGF family, which are non-heparin binding, sHB-EGF is able to bind to cell-surface heparin-like molecules (heparan sulfate proteoglycans; HSPG), which act as low affinity, high capacity receptors for HB-EGF. The differing affinities of EGF family members for the different EGFR subtypes and for HSPG may confer different functional capabilities to these molecules in vivo. The combined interactions of HB-EGF with HSPG and ErbB-1/ErbB-4/NRDc may confer a functional advantage to this growth factor.
Although the HB-EGF gene is widely expressed, the basal level of its mRNA is relatively low in normal cells. Expression of HB-EGF is significantly increased in response to tissue damage, hypoxia and oxidative stress, and also during wound healing and regeneration. This pattern of expression is consistent with a pivotal role for HB-EGF in ischemia/reperfusion (I/R) injury, regeneration, and repair processes.
Intestinal barrier function represents a critical initial defense against noxious intraluminal substances. Although the intestine is not as essential as the vital organs in the immediate preservation of life, I/R is as lethal as extensive heart and brain ischemia. The gut has a higher critical oxygen requirement compared to the whole body and other vital organs. Accordingly, the intestinal mucosa is extremely susceptible to I/R and even short periods of ischemia can initiate local and remote tissue damage as well as systemic hemodynamic disturbances.
Reactive oxygen species (ROS), pro-inflammatory cytokines, leukocyte adhesion, and complement activation can all mediate intestinal I/R. Loss of immune and barrier functions of the gut secondary to I/R leads to significant detrimental effects on other organs such as lungs, liver, kidneys and heart, and may result in multiple organ dysfunction syndrome (MODS) and death. Exploring the potential of new therapeutic strategies to enhance the regenerative capacity and/or increase the resistance of the intestine to I/R injury would improve outcome in these patients.
The gut is highly susceptible to hypoperfusion injury due to its higher critical oxygen requirement compared to the whole body, and due to the mucosal countercurrent microcirculation. Not surprisingly, patients subjected to hypoperfusion states such as hemorrhagic shock and resuscitation (HS/R), trauma, and major surgery often develop intestinal ischemia as documented by both experimental and clinical studies.
Following the hypoperfusion effects of the shock stage, traditional methods of resuscitation often fail to adequately restore mesenteric perfusion despite stabilization of heart rate, blood pressure, and improved perfusion in some organs such as the heart and brain. To the contrary, resuscitation is characterized by progressive deterioration of mesenteric blood flow. Progressive intestinal hypoperfusion after HS/R contributes to loss of the gut mucosal barrier and to hypoxia-induced intestinal inflammation, both of which are critical to the initiation of MODS after HS/R. Accordingly, factors that protect the intestine from injury and promote early intestinal healing by restitution could significantly improve outcome after HS/R.
HB-EGF has been demonstrated to be essential for intestinal healing by restitution in intestinal epithelial cells (IEC) in vitro and in rats subjected to superior mesenteric artery occlusion (SMAO) in vivo (El-Assal & Besner Gastroenterology 129(2): 609-625. 2005). These HB-EGF-induced effects are mediated via activation of various molecular mechanisms including MEK/ERK and PI3K/Akt signaling pathways.
A few reports have demonstrated the importance of HB-EGF in promoting endothelial cell (EC) functions including angiogenesis. Exogenous HB-EGF was shown to promote rabbit corneal angiogenesis and neovascularization in mouse skin (Abramovitch et al., FEBS Lett 425(3):441-7, 1998) and recent studies have shown that HB-EGF is involved in tumor angiogenesis (Ongusaha et al. Cancer Res 64(15):5283-90, 2004), catecholamine-induced vascular trophic effects (Zhang et al., Circ Res 95(10):989-97, 2004) and angiopoietin-induced angiogenesis (Iivanainen et al., Faseb J 17(12):1609-21, 2003). HB-EGF belongs to the epidermal growth factor (EGF) family that functions via activation of the tyrosine kinase EGF receptor (EGFR). Another member of this family, EGF itself, has been shown to increase mesenteric blood flow in sheep and to induce direct relaxation of isolated rabbit mesenteric arteries via activation of EGFR. Further studies have demonstrated that the gastroprotective effects of EGF are mediated in part by its ability to increase gastric blood flow. To date, the effects of HB-EGF on mesenteric blood flow have not been elucidated.
Hemorrhagic shock (HS)-induced injuries and hemorrhagic shock and resuscitation (HS/R)-induced injuries are conditions that may result from any type of trauma or severe blood loss. Therefore, there is a continuing need to develop methods of preventing injuries due to HS or HS/R and therapeutic compositions for preventing and treating these intestinal injuries.